1. Field of the Invention
The present invention relates to an ink for use in ink-jet recording that can be applied for producing color filters provided with at least green pixels, a color filter, a liquid crystal panel and a computer, and a method for producing the color filter.
2. Related Art
Color filter substrates are important elements of color liquid crystal displays having many colored portions comprising, for example, the three primary colors of R (red), G (green) and B (blue), which are aligned on a substrate with a given pattern.
With the recent development of personal computers, especially of portable personal computers, demands on liquid crystal displays, especially demands on color liquid crystal displays, are increasing. Moreover, it will be necessary to satisfy the two competing demands of reducing the production cost of the display unit on the one hand while providing displays with finer and better image qualities on the other hand. Especially, the demands for color filters relating to production cost are rapidly increasing.
Although a variety of methods have been attempted to comply with the foregoing demands while satisfying the required characteristics of the color filter, no method that can satisfy all the required characteristics has been established yet.
Representative methods for producing color filters will be described below.
The first, most popular, method is a dyeing method. In the dyeing method, a composition prepared by adding photosensitive agents to a water soluble polymer material having affinity for dyeing is used. After forming a receptor layer by patterning the composition into a desired configuration on a substrate by a lithographic process, the receptor layer is immersed in a dyeing bath to obtain a colored portion. This process is repeated three times, thereby obtaining a color filter for the three colors of R, G and B.
The second frequently used method is a pigment dispersion method, which is currently replacing the dyeing method. In this method, a photosensitive resin layer in which pigments are dispersed is at first formed on the substrate and a monochromatic pattern is obtained by patterning the photosensitive resin layer. This process is repeated three times, thereby obtaining a color filter for the three colors of R, G and B.
The third method is a electrodeposition method. In this method, a transparent electrode is patterned on the substrate. Then, the patterned transparent electrode is immersed in an electrodeposition coating solution prepared by dispersing or dissolving pigments, a resin and an electrolyte to subject the electrode to electrodeposition with the first color. This process is repeated three times followed by baking, thereby obtaining a color filter for the three colors of R, G and B.
The fourth method is a printing method. In the printing method, pigments of a heat-curing type resin are dispersed and printing is performed three times followed by heat-curing the resin, thereby obtaining a color filter for the three colors of R, G and B.
A protective layer is usually formed on the colored layer in any of the methods described above.
The common step in these methods is to repeat the same step three times for coloring with the three colors of R, G and B, which naturally increases the production cost. It is also a problem that the more numerous the production steps, the more the production yield decreases.
In addition, the electrodeposition method cannot be applied to the TFT (Thin Film Transistor) type color liquid crystal display with the currently available technology since the configuration of patterns able to be formed by the electrodeposition method is limited. There is a problem in the fourth method--the printing method--that resolution of images and smoothness are poor, making it impossible to form patterns with a fine pitch.
Though there are many methods for producing color filters as described above, the dyeing method using dyes as coloring materials is generally considered to be advantageous when emphasis is put on the coloring ability of the display.
However, in the dying method, it is quite important to find out dyes having not only good color tones, i.e. preferable spectroscopic characteristics, but also superior dyeing abilities in order to effectively stain a dye-receiving layer, and at present, we cannot find out dyes which satisfies with the requirements in high level. Moreover, when one attempts to obtain a desired color tone (toning) by mixing two or more of the dyes, irregular colors are liable to be generated because the different dyes have different dyeing abilities with respect to the receptor layer materials, making it very difficult to control the color tone. This phenomenon becomes especially evident when the structures of the dyes to be mixed are different from each other. When phthalocyanine dyes are used for forming G or B colored portions, for example, it is a problem that other dyes that can be used together with those dyes for toning were severely limited, due to the poor fixing ability of the phthalocyanine dyes. In order to avoid the foregoing problem, it was attempted to generate the green color with one molecular structure by allowing a compound having a mono-azo structure (yellow color component) to combine with a compound having a phthalocyanine frame (cyan color component), thereby obtaining a desired color tone without toning. However, while the problem of color irregularity is minimized with this method since no color toning is required, there is an another problem, namely, that it is difficult to improve transmittance.
In order to improve the fixing ability of the dye to the receptor layer described above, introducing cation groups such as a quaternary ammonium group in the receptor layer is usually attempted when the dye is an anion type dye. However, other problems of change of the color tone of the dye (spectral shift) or deterioration of heat resistance have appeared.
For solving the foregoing problems, a method for producing color filter substrates using an ink-jet method has been proposed (see, for example, Japanese Unexamined Patent publications No. 59-75205, No. 63-235901, No. 1-217302 and No. 4-123005). In contrast to the methods described above, the respective colored portions of R, G and B are formed by one step. These methods comprise, for example, a method for directly attaching a curing type ink containing each of the color materials followed by curing, and a method for coloring the ink receptor layer by directly printing with the coloring inks.
Means for improving fixing ability such as introducing a cation group into the receptor layer are not required in forming colored portions in the ink-jet method described above since a fixing process between the dye and receptor layer as used in the forgoing dyeing method is not needed. Accordingly, it is possible to avoid color tone change (spectral shift) of the dye itself before and after dyeing or a decrease in heat resistance, further eliminating the possibility that the color tone will be very different from the expected color tone, even when an ink containing a plurality of dyes is used for toning.
Colored portions colored with two or more colors can be formed by one step in the ink-jet method, enabling improved productivity and reduced production cost because there is no waste of used ink.
While it is preferable that pixels in the color filter have as high a color reproducibility and light permeability as possible, these two characteristics are often incompatible with each other. For example, although a variety of green dyes or pigments, or dyes in which a yellow dye and a cyan dye are mixed, were investigated and used for green pixels for use in a color filter, there were problems: the light permeability decreased when the coloring concentration of the pixels increased for attaining a high color reproducibility, or the color reproducibility became poor when an emphasis was put on light permeability. Therefore, technical developments for obtaining green pixels in which a high level of compatibility between both color reproducibility and light permeability is realized are urgently desired.